docs: def get_value_for_datastore(self, model_instance, cached=False):
          """Get key of reference rather than reference itself."""
          value = getattr(model_instance, self._attr_name(), None)
          if not cached:
              value = getattr(model_instance, self.property_name).keys()
              setattr(model_instance, self._attr_name(), value)
          return value